Gets the namespace the element is in: its NS namespace dictionary, or Pdf17, the
default namespace, when it has none.
public PdfStructureNamespace Namespace { get; }
ISO 32000-2 §14.7.4, Table 355 (NS, PDF 2.0) and §14.8.6.2: a namespace an element uses shall be listed in the root's
Namespaces array; one that is not is still used, with a NamespaceNotDeclared diagnostic.